Currently, the electrodes applied for stimulating nerve fibers are unable to be made as small as individual nerve fibers. In prior arts, the nerves in the proximity of the electrodes can be stimulated, but some nerves located between the electrodes are unable to be stimulated. Because the number of electrodes determines the number of perceptual channels in prior arts, it not only leads to low stimulation resolution, but also fails to stimulate some of the nerves.
The electrodes, mediums for delivering electrical charges to stimulate nerve fibers and to reconstruct a normal hearing transmission pathway, often fail to cover and clearly distinguish all the nerve fibers due to its physical dimension. Moreover, the stimulation resolution and the actual perceived information is degraded. For example, even if 16-22 electrodes are implanted into a cochlear implant user, there are approximately 30,000 auditory nerve fibers in a typical normal ear and less for a typical hearing impaired ear and only auditory nerve fibers in the proximity of the electrodes could be stimulated. Therefore, the hearing perception quality of cochlear implant users is degraded due to low stimulation coverage and low spatial stimulation resolution. At present, there are a certain number of languages can afford lower stimulation resolution and bandwidth in relatively quiet environment, the cochlear implant is able to assist patients using such languages. However, for tonal languages such as the Mandarin Chinese and Cantonese Chinese, the cochlear implant users often cannot clearly distinguish the tones of the words and understand what they hear. More importantly, music cannot be fully captured by a typically cochlear implant system and noisy environment would impede the users' hearing quality further.
Generally, electrical stimulation generated by an electrode is carryout by applying a current or voltage to at least one physical electrode, so as to generate an electrical stimulation signal in the electrode, and subsequently stimulate nerve fibers in the proximity of the electrode. Recently, it is proposed to use the so-called virtual electrode or virtual channel technology to improve the hearing resolution without the need to increase the original number of implanted electrodes. The generation of the virtual electrode or virtual channel is mainly achieved by adjusting the current ratio of at least two electrodes, so as to generate a stimulation signal between the two electrodes. In other words, current levels of two adjacent electrodes are adjusted by current control so as to generate an intermediate channel (stimulation) between the two adjacent electrodes. Since no physical electrode is present between the two electrodes, the generated intermediate stimulation signal is called the virtual electrode or virtual channel. Therefore, it is possible to generate electrical stimulation signal and stimulation sites that exceeds the number of the physical electrodes, so as to stimulate more nerve fibers located between the electrodes, thereby improving the resolution of stimulation and the hearing quality of cochlear implant users. Moreover, the power sources of the electrodes can be configured as current sources or voltage sources.
The virtual channel technology is mainly achieved by adjusting a ratio of power setting of two electrodes and can be found in U.S. Patent Publication No. 2004/0136556 published on Jul. 15, 2004. In U.S. Patent Publication No. 2004/0136556, it is disclosed that a plurality of electrodes of an identical power polarity are actuated by adjusting a current ratio of the electrodes to a reference electrode or a ground electrode of an inverse power polarity with respect to the electrodes of the identical power polarity for balancing a total power polarity. This type of stimulation is called monopolar stimulation. However, the beamwidth of the electrical stimulation signal resulted from monopolar stimulation is so wide that it is difficult to focus the stimulation, and the number of distinguishable virtual channels is not sufficient to improve the resolution of hearing frequency.
In the April issue of IEEE Transactions on Biomedical Engineering of 1999, a paper titled “Proposal of a New Method for Narrowing and Moving the Stimulated Region of Cochlear Implants: Animal Experiment and Numerical Analysis” was proposed by Shigeki et al. In summary, Shigeki and colleagues proposed a method that uses a virtual electrode generated by three adjacent electrodes (wherein a total current of two lateral electrodes is equal to the current of the central electrode) to narrow down stimulation regions, and a stimulation location is moved by adjusting parameters of the two lateral electrodes. This method is capable of narrowing down the stimulation signal (beamwidth) by adjusting the parameters of the two lateral electrodes. However, by this type of electrical stimulation apparatus, only the nerve fibers in the proximity of the central electrode is able to be stimulated, that is, the apparatus has an effective stimulation range restricted to the vicinity of the central electrode and is incapable of sufficiently moving the location of the virtual channel or stimulation sites between the electrodes. As a result, this virtual electrode cannot be used to stimulate the nerve fibers located between the two electrodes, and hence the stimulation resolution of the signal is not improved effectively.
Although the prior art US2004/0136556 had implied an idea of combining multiple electrodes for generating a virtual channel, no specific implementation was proposed, and the use of multiple electrodes for adjusting the virtual channel would be technically more complicated than using only two electrodes. In addition, regardless of how many electrodes are used to generate the virtual channel in the prior art US2004/0136556, an additional external reference electrode (or grounding electrode) is still required to balance the total power polarity of the electrode set. It is costly, and the efficiency of electrodes usage would be degraded.